As is known, the obtention of metallic parts by means of molding, comprises the pouring of cast metal into a mold, the solidification of the metal through cooling and the demolding or extraction of the formed part, by means of the removal or destruction of the mold.
Said molds may be metallic or may be formed by aggregates of different materials (ceramics, graphites and especially, sand), normally hardened by the action of agglomerates. Generally, the sand molds are obtained by filling a molding die with sand.
Said molds shall be equipped with gates or orifices for the communication between the internal and the external cavity, through which the cast metal in molding or casting form, is poured. Likewise, due to the shrinkage of the metal during the cooling, the mold shall be provided with vertical cavities or flash channels which are filled with reserve cast metal with the object of forming a feeding head intented to compensate the shrinkage or drawing of the metal.
The purpose of the feeding head is to supply the part when the medium is shrunk in the same, due to which the metal shall be kept in the feeding head in liquid condition a longer time than the part. For this reason, the flash channels are normally covered with ferrules manufactured with isothermic or even exothermic refractory materials (insulations) which delay the cooling of the metal contained in the feeding heads in order to ensure its fluidity when the drawing in the cast metal is produced.
The gates through which the cast metal is poured are also constructed from refractory, insulating and even exothermic materials, with similar composition to that of the ferrules.
Suitable insulation refractory compositions are known for the production of ferrules and other feeding head and supply elements for casting molds, with insulating properties, constructed from a refractory material in the manner of particles, organica and/or inorganic fibers and agglomerantes.
Sutiable exothermic refractory compositions are also known for the production of ferrules and other heeding head and supply elements for casting molds, with exothermic properties, comprised of a refractory filler material in the form of fibers or particles, agglomerants and, optionally, selected loads from among an easily oxidizable metal and an oxidant agent, capable of oxidizing said metal. Additionally, in order to improve the sensitivity of the exothermic refractory composotion, an inorganic fluorine flux is generally included. British Patentes Nos. GB 627678, 774491, 889484 and 939541 disclose exothermic refractory compositions which contain inorganic fluorides.
Additionally, the PCT application, published with International Publication Number WO94/23865, discloses a composition for a casting mold of metals which comprises hollow micro beads containing alumina, in which the alumina content is at least, 40% in weight.
The great majority of the ferrules which are consumed at world level are manufactured by vacuum and wet molding, followed by drying and polymerization of the resins at high temperature, such as is mentioned in Spanish Patent N.degree. ES-8403346. A standard procedure of this type comprises the stages of:
the suspension in water of a mixture formed by the materials used in the manufacturing of the ferrule, for example, aluminosilicate fibers, aluminum, iron oxide and phenolic resins, or alternatively, a mixture formed by siliceous sands, aluminum scoria, cellulose, aluminum and phenolic resins; PA1 the aspiration of said aqueous suspension by means of vacuum through an exterior and interior mold; and PA1 the demolding of a green or wet ferrule, deposited on a tray, which in turn is introduced into an oven in which it remains between 2 and 4 hours at a temperature of approximately 200.degree. C., and finally, left to cool. PA1 the impossibility of obtaining ferrules with the sufficient external dimensional exactitude, since the aspiration of the mixture through the mold produces a good exactness of the ferrule on the internal face (the one which is in contact with the mold) but not of the other face. This inexactitude makes the external contour of the ferrules not coincide dimensionally with the internal cavity of the flash channels, often originating important difficulties for its placement and attachment. Even when there is a double mold, it is difficult to keep to the measurements due to its subsequent handling in green condition. In this sense, techniques have been developed for the placement of the ferrules in their housing, such as is disclosed in German Patent N.degree. DE P 29 23 393.0; PA1 it requires long production times; PA1 it presents difficulties in the homogeneization of the mixtures; PA1 it impossibilitates the introduction of rapid changes in the formulation; PA1 it presents certain hazards during the manufacturing process and polution of residual waters; and PA1 the materials used in the form of fibers may cause alergical pathologies, such as itching, and skin and mucous irritation, to the operators.
On occasions, all the aluminosilicate stock material is not found in the form of fibers since a part of the same may have been replaced by hollow micro beads of said aluminosilicate material with the object of decreasing the necessary quantity of product and reducing the cost of the final product. Such micro beads are then used as loading element.
This procedure permits the obtention of insulating or exothermic ferrules, but it presents numerous disdadvantages, among which the following are to be found:
Another procedure for the manufacturing of ferrules consists in mixing sand, exothermic materials and a specific type of resin, for example, mixing sodium silicate and alkaline or novolac phenolic resins, and subsequently, performing a manual or blow molding of the obtained mixtures. With said procedure, parts of great dimensional exactitude may be obtained, both internal and external, with exothermic properties, though never with insulating properties. Though this procedure is simpler that the wet means, its employment presents serious limitations since, on one hand, it is not possible to obtain ferrules with insulating characteristics and, on the other hand, the ferrules obtained are extraodinarily hygroscopic.
Finally, Application WO94/23865 discloses a blowable composition based on aluminium silicate hollow micro beads, though requiring that the alumina content of the same be over 40.sup.a, which makes unusable a significant part of said by product, because a very important part of the aluminium silicate hollow micro beads generated as industrial by product, have a lower richness than the 40% weight in alumina.
As may be appreciated, a procedure exists for the manufacturing of ferrules by wet means and vacuum molding which provides ferrules equipped with insulating or exothermic properties, though with dimensional inexactitude, the development of which presents numerous disadvantages, and on the other hand, there exists a simpler production procedure of ferrules by dry means and manual or blow molding, though only permitting the obtention of ferrules provided with exothermic properties, not insulation, but with dimensional exactness.
It would be very desirable to have ferrules and other feeding head and supply elements provided with insulating or exothermic properties, which would present dimensional exactness, and which, additionally, could be manufactured by means of a simple procedure which would overcome the previously indicated disadvantages as regards the known procedures. The invention provides a solution to said problems which comprises the use of a refractory material, such as aluminium silicate, in the form of hollow micro beads with an alumina content below 38% in weight, in the formulation of a suitable composition for the production of said ferrules and feeding head and supply elements for casting molds.
Consequently, an object of this invention is constituted by the use of aluminium silicate hollow micro beads with an alumina content below 38% in weight in the formulation of a composition which is totally exent of refractory, insulating or exothermic material, in the form of fibers, suitable for the manufacturing of ferrules and other feeding head and supply elements for casting molds, insulating or exothermic.
Another object of the invention is constituted by a suitable formulation for the manufacturing of ferrules and other feeding head and supply elements for casting molds, which comprises aluminium silicate, hollow micro beads with an alumina content below 38% in weight, an agglomerant and optional loads. The ferrules and other feeding head and supply elements, manufactured parting form the previously mentioned formulation, which may be insulating or exothermic, as well as their manufacturing procedure, constitute additional objects of this invention.
On the other hand, industrial experience in nodular casting manifests that in parts with a silicon content equal to, or over 2,8%, a thickness over 20 mm and a fluorine content in green sand over 300 ppm (parts per million), a reaction takes place causing in the parts whitish pores which makes them unserviceable.
The fluorine causing the rejection of the parts may come from the bentonite, the water or the sand, but, mainly, from the fluoride derivates used in the composition for the obtention of exothermic ferrules, because of which, if said ferrules are used extensively, the circuit of green sand may be made to reach undesirable limits in fluorine contents.
Therefore, it would be very desirable that the ferrules and other suitable exothermic elements for the nodular casting should not contribute fluorine, or that the fluorine contributions should be very reduced. The invention offers a solution to said problem which comprises the employment of an insert, the composition of which contains an inorganic fluorine flux, in the manufacturing of ferrules and exothermic feeding head and supply elements suitable for nodular casting, and which is fixed on a zone of said ferrules and elements.
Consequently, an additional object of this invention is constituted by a procedure for the production of ferrules and exothermic feeding head and supply elements, suitable for nodular casting which comprises the formation and attachment of an insert made up of an inorganic fluorine flux, over a formed composition, forerunner of said ferrule or element constituted by aluminium silicate hollow micro beads with an alumina content below 38% in weight, an agglomerant and optional loads.